A bicycle is normally provided with two gearshifts, a front one associated with the crankset and a rear one associated with the sprocket set. In both cases, the gearshift comprises a derailleur which engages the drive chain moving it on toothed wheels with different diameters and numbers of teeth, so as to obtain different transmission ratios; the derailleur, be it that of the rear gearshift or that of the front gearshift, is moved in a direction by a traction action applied by an inextensible cable that is normally sheathed (commonly known as Bowden cable), in the opposite direction by the elastic return action of a spring provided in the gearshift itself. Normally, the direction in which the displacement is determined by the return spring is that in which the chain passes from a toothed wheel of greater diameter to a toothed wheel of smaller diameter, i.e. that of so-called downward gearshifting; vice-versa, the traction action of the control cable takes place in the direction of so-called upward gearshifting, in which the chain moves from a toothed wheel of smaller diameter to a toothed wheel of greater diameter. It should be noted that in a front gearshift downward gearshifting corresponds to the passage to a lower transmission ratio, whereas in a rear gearshift it corresponds to a greater transmission ratio.
The displacement in the two directions of the control cable of a gearshift is obtained through an actuation device mounted so as to be easy to operate by the cyclist, i.e. normally on the handlebars, near to its grips. By convention, near to the left grip there is the actuation device of the control cable of the front gearshift and vice-versa near to the right grip there is the actuation device of the control cable of the rear gearshift.
In the actuation device, the control cable is actuated into traction or into release through winding and unwinding on a rotor element, commonly known as cable-winding bobbin, the rotation of which is controlled by the cyclist with different means according to the type of gearshift. In a typical configuration, the actuation device provides two distinct ratchet gear levers, to control the rotation in the two directions of the bobbin.
In any case, the actuation device must provide that the bobbin be held still in rotation in a number of predetermined angular positions, corresponding to the different positions of the derailleur required by the different ratios, i.e. on the different toothed wheels of the gearshift. Thus, the actuation device of a front gearshift has a relatively low number of predetermined angular positions (typically three), corresponding to the positions of the front derailleur on the different crowns of the crankset; a rear gearshift, on the other hand, has a relatively high number of predetermined angular positions (typically from seven to ten), corresponding to the positions of the rear derailleur on the different sprockets.
The holding in the predetermined positions is obtained by so-called indexer assembly, which must ensure that the cable-winding bobbin is held sufficiently strongly to keep the set position with precision even in the presence of knocks and bangs during ride, but at the same time also sufficiently yielding to allow the passage from one position to the other when the cyclist actuates the levers to change ratio.
Fundamentally, two types of actuation devices are known, those known as release and active devices.
In release actuation devices, the rotation of the cable-winding bobbin in the winding direction of the cable takes place thanks to the physical force applied by the cyclist on the appropriate lever, whereas the rotation in the opposite direction is obtained substantially by freeing the cable-winding bobbin and letting the return spring on the gearshift act in the direction to unwind the cable from the bobbin, clearly in a controlled manner. In these devices, therefore, during upward gearshifting, the cyclist must first overcome the holding action of the indexer assembly, then the return action of the spring of the derailleur, until the gearshifting is completed; during downward gearshifting, on the other hand, the cyclist must only overcome initially the holding action of the indexer assembly, whereas the return action of the spring of the derailleur is of help during the entire gearshifting. The return action of the spring of the derailleur, however, comes from an elastic-type force, and is therefore different according to the position; in particular, it is greater towards the high gear positions and smaller towards the low gear positions (by high and low gears meaning not long or short ratios, but rather ratios at which one arrives with upward gearshifts or with downward gearshifts). This determines an operative asymmetry that may be unwelcome for the cyclist, especially in demanding conditions of use like during a race.
To remedy the drawbacks of release devices, active devices have been developed, in which the cyclist is required to apply an active force during the entire gearshifting, be it downwards or upwards. In such a device, an elastic member is provided that counters the return action of the spring of the derailleur. The action of such elastic means is balanced with that of the spring of the derailleur, so that—if as a hypothesis the indexer assembly were missing and the only actions on the bobbin were those of the elastic means and of the return spring—the cable-winding bobbin would be arranged in an intermediate position, thus corresponding to an intermediate ratio of the gearshift.
However, such a device does not totally solve the problem. Indeed, with it gearshifting (upward or downward) moving away from the intermediate ratio are countered, whereas gearshifting (upward or downward) moving towards the intermediate ratio are facilitated.
There is therefore the problem of making the force required of the cyclist more homogeneous for different gearshifting.